U.S. patent application number 10/107977 was filed with the patent office on 2003-10-02 for adaptive digital sub-array beamforming and deterministic sum and difference beamforming, with jamming cancellation and monopulse ratio preservation.
Invention is credited to Yu, Kai Bor.
Application Number | 20030184473 10/107977 |
Document ID | / |
Family ID | 28452760 |
Filed Date | 2003-10-02 |
United States Patent
Application |
20030184473 |
Kind Code |
A1 |
Yu, Kai Bor |
October 2, 2003 |
Adaptive digital sub-array beamforming and deterministic sum and
difference beamforming, with jamming cancellation and monopulse
ratio preservation
Abstract
A radar system and technique provide the capability to detect a
target of interest and maintain the detection in the presence of
multiple mainlobe and sidelobe jamming interference. The system and
technique utilize the versatility of digital beamforming to form
sub-arrays for canceling jamming interference. Jamming is
adaptively suppressed in the sub-arrays prior to using conventional
deterministic methods to form the sum, .SIGMA., and difference,
.DELTA., beams for monopulse processing. The system and technique
provide the ability to detect a target of interest, provide an
undistorted monopulse ratio, m, and maintain target angle
estimation, in the presence of multiple mainlobe and multiple
sidelobe jammers. Further, this system and technique are not
constrained by requiring a priori knowledge of the jamming
interference.
Inventors: |
Yu, Kai Bor; (Niskayuna,
NY) |
Correspondence
Address: |
DUANE MORRIS LLP
Suite 100
100 College Road West
Princeton
NJ
08540
US
|
Family ID: |
28452760 |
Appl. No.: |
10/107977 |
Filed: |
March 27, 2002 |
Current U.S.
Class: |
342/380 |
Current CPC
Class: |
G01S 7/2813 20130101;
G01S 2013/0245 20130101; G01S 13/4463 20130101 |
Class at
Publication: |
342/380 |
International
Class: |
G01S 003/16 |
Claims
What is claimed is:
1. A method for detecting a radar target of interest in the
presence of radar jamming interference, said method comprising the
steps of: forming a plurality of sub-arrays from an antenna array;
forming a respective sub-array beam pattern for each of said
plurality of sub-arrays; wherein at least one null of each
sub-array beam pattern is steered toward an interference; and
determining a sum beam from said sub-array beam patterns for
detecting said target of interest.
2. A method in accordance with claim 1, further comprising the
steps of: determining at least one difference beam from said
sub-array beam patterns; and determining at least one monopulse
ratio from said sum and at least one difference beams for
estimating an angle of arrival of said target of interest.
3. A method in accordance with claim 2, wherein difference beams
are determined in azimuth and elevation.
4. A method in accordance with claim 1, wherein said sub-array beam
patterns are formed having a constraint of maintaining a boresight
gain of each sub-array beam pattern.
5. A method in accordance with claim 1, wherein said step of
forming said sub-array beam patterns comprises adaptively forming
said sub-array beam patterns.
6. A method in accordance with claim 1, wherein said step of
forming said sub-array beam patterns comprises digitally
beamforming said sub-array beam patterns.
7. A method in accordance with claim 1 further comprising the step
of maintaining an estimated angle of arrival of said target of
interest, said step of maintaining comprising: updating each
sub-array beam pattern to steer said at least one null of each
sub-array beam pattern toward interference and to maintain a
boresight gain of each sub-array beam pattern; determining updated
sum and difference beams from said updated sub-array beam patterns;
and determining at least one updated monopulse ratio from said
updated sum and difference beams for maintaining an estimated angle
of arrival of said target of interest.
8. A radar system for detecting a radar target of interest in the
presence of interference, said system comprising: an antenna array
for providing antenna array element data; a summer for forming a
plurality of sub-arrays from said antenna array element data; a
sub-array beamformer for forming a respective sub-array beam
pattern from each of said plurality of sub-arrays; wherein at least
one null of each sub-array beam pattern is steered toward an
interference; a monopulse summer for forming sum beams from said
sub-array beam patterns for detecting said target of interest.
9. A radar system in accordance with claim 8, further comprising: a
monopulse difference beamformer for forming at least one difference
beam from said sub-array beam patterns; and a monopulse ratio
former for determining at least one monopulse ratio from said sum
and at least one difference beams for estimating an angle of
arrival of said target of interest.
10. A system in accordance with claim 9, wherein difference beams
are determined in elevation and azimuth.
11. A system in accordance with claim 8, wherein said sub-array
beamformer is a digital beamformer.
12. A system in accordance with claim 8, wherein said sub-array
beam patterns are formed having a constraint of maintaining a
boresight gain of each sub-array beam pattern.
13. A system in accordance with claim 8, wherein said sub-array
beamformer is an adaptive sub-array beamformer for adaptively
forming said sub-array beam patterns.
14. A system in accordance with claim 13, wherein said plurality of
sub-arrays comprises four sub-arrays each sub-array formed from a
different quadrant of said antenna array.
15. A system in accordance with claim 14, wherein each sub-array
beam pattern is formed in accordance with the following equations:
10 Q ^ 1 ( T x , T y ) = Q ^ ( T x , T y ) j2 ( T x D x + T y D y )
, Q ^ 2 ( T x , T y ) = Q ^ ( T x , T y ) j2 ( - T x D x + T y D y
) , Q ^ 3 ( T x , T y ) = Q ^ ( T x , T y ) j2 ( - T x D x - T y D
y ) , Q ^ 4 ( T x , T y ) = Q ^ ( T x , T y ) j2 ( T x D x - T y D
y ) , wherein, D.sub.x is a distance in azimuth between a center of
a respective sub-array and a center of said antenna array; D.sub.y
is a distance in elevation between a center of a respective
sub-array and said center of said antenna array; .lambda. is a
wavelength of transmitted radar energy; T.sub.x is a directional
cosine representing azimuth with respect to said center of said
antenna array; T.sub.y is a directional cosine representing
elevation with respect to said center of said antenna array;
{circumflex over (Q)}.sub.1(T.sub.x,T.sub.y) is an estimated first
quadrant beam located at (D.sub.x,D.sub.y) with respect to said
center of said antenna array; {circumflex over
(Q)}.sub.2(T.sub.x,T.sub.y) is an estimated second quadrant beam
located at (-D.sub.x,D.sub.y) with respect to said center of said
antenna array; {circumflex over (Q)}.sub.3(T.sub.x,T.sub.y) is an
estimated third quadrant beam located at (-D.sub.x,-D.sub.y) with
respect to said center of said antenna array; {circumflex over
(Q)}.sub.4(T.sub.x,T.sub.y) is an estimated fourth quadrant beam
located at (D.sub.x,-D.sub.y) with respect to said center of said
antenna array; and {circumflex over (Q)}(T.sub.x,T.sub.y) is an
estimated common sub-array factor located at said center of said
antenna array.
16. A system in accordance with claim 15, wherein a calculation of
monopulse ratios results in the following equations: 11 m ^ A = j
tan ( 2 T x D x ) , m ^ E = j tan ( 2 T y D y ) , wherein,
{circumflex over (m)}.sub.A is an estimated monopulse ratio in
azimuth; and {circumflex over (m)}.sub.E is an estimated monopulse
ratio in elevation.
17. A computer readable medium having embodied thereon a computer
program for detecting a radar target of interest in the presence of
a interference, the computer readable program comprising: means for
causing a processor to form a plurality of sub-arrays from an
antenna array; means for causing said processor to adaptively form
a respective sub-array beam pattern for each of said plurality of
sub-arrays, wherein at least one null of each sub-array beam
pattern is steered toward an interference and a boresight gain of
each sub-array beam pattern is maintained; means for causing said
processor to determine sum and difference beams from said sub-array
beam patterns; and means for causing said processor to determine at
least one monopulse ratio from said sum and difference beams for
detecting a radar target of interest and for estimating an angle of
arrival of said target of interest.
18. A computer readable medium in accordance with claim 17, further
comprising means for maintaining an estimated angle of arrival of
said target of interest, said means for maintaining comprising:
means for updating each sub-array beam pattern to steer said at
least one null of each sub-array beam pattern toward interference
and to maintain a boresight gain of each sub-array beam pattern;
means for determining updated sum and difference beam patterns from
said updated sub-array beam patterns; and means for determining at
least one updated monopulse ratio from said updated sum and
difference beams for maintaining an estimated angle of arrival of
said target of interest.
Description
FIELD OF THE INVENTION
[0001] This invention generally relates to radar systems and
techniques for determining the angular location of a target and
specifically to a monopulse radar processing system and technique
for maintaining the accuracy of the monopulse ratio in the presence
of multiple mainlobe jammers and multiple sidelobe jammers.
BACKGROUND
[0002] One of the problems facing surveillance and fire control
radar systems today is target detection and estimation of target
angle in the presence of severe jamming interference. This problem
is particularly important for next generation radar systems used in
missile defense. Recently, interest has been generated toward a
goal of implementing radar systems in airborne and spaceborne
platforms for large area surveillance. A problem associated with
achieving this goal is developing a radar system capable of
detecting targets while rejecting unwanted information such as
jammers and clutter.
[0003] Radar systems implementing antenna arrays typically form
beam patterns comprising a central beam, i.e., main lobe, and
surrounding minor lobes, i.e., sidelobes. Typically, it is desired
to have a narrow mainlobe having high gain, and low sidelobes. To
detect a desired target and reject unwanted clutter and jamming,
the mainlobe is steered toward the target of interest. The desired
target within the mainlobe is enhanced and the response to clutter
and jamming outside the mainlobe is attenuated. However, if a
jammer is located within the mainlobe, it becomes difficult to
detect the target of interest. This problem is exacerbated in the
situation where multiple jammers exist.
[0004] Radar systems have been developed to cancel a single jammer
in the mainlobe. Such a system is described in U.S. Pat. No.
5,600,326 issued to Yu et al., which is incorporated herein by
reference in its entirety. However, these systems require a priori
knowledge of the jammer location. Thus, a need exists for a radar
system having the ability to detect a target of interest in the
presence of multiple mainlobe jammers. A need also exists for a
radar system having the capability to cancel multiple mainlobe
jammers without requiring a priori knowledge of jammer locations.
Further, a need exists for a radar system having the capability to
detect a target of interest in the presence of multiple mainlobe
and multiple sidelobe jammers.
SUMMARY OF THE INVENTION
[0005] A radar system for detecting and maintaining a detection of
a target of interest in the presence of interference includes an
antenna array, a summer, a sub-array beamformer, and a monopulse
processor. Antenna array element data are provided to the summer to
form a plurality of sub-arrays. The sub-array data are provided to
the sub-array beamformer for forming respective sub-array beam
patterns. The monopulse processor includes a monopulse
sum-difference beamformer and a monopulse ratio former. The
monopulse sum-difference beamformer forms sum and difference beams
from the sub-array beam patterns, and the monopulse ratio former
forms at least one monopulse ratio from the sum and difference
beams.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] The foregoing and other objects, aspects and advantages will
be better understood from the following detailed description of a
preferred embodiment of the invention with reference to the
drawings, in which:
[0007] FIG. 1 is a block diagram of an exemplary monopulse
sub-array sum-difference beamformer;
[0008] FIG. 2 is a block diagram of an exemplary adaptive digital
sub-array beamformer and monopulse sum and difference beamformer,
with jamming cancellation and monopulse ratio preservation, in
accordance with the present invention;
[0009] FIG. 3 is a block diagram of an exemplary adaptive digital
sub-array beamformer and monopulse sum and difference beamformer,
with jamming cancellation and monopulse ratio preservation, for a
one-dimensional array, in accordance with the present
invention;
[0010] FIG. 4 is a flow diagram of an exemplary process for
detecting a target of interest in the presence of multiple mainlobe
and sidelobe jammers, in accordance with the present invention;
and
[0011] FIG. 5 is a block diagram of an exemplary radar system in
accordance with the present invention.
DETAILED DESCRIPTION
[0012] Monopulse radar processing is a radar processing technique
in which the angular location of a target (also referred to as
direction of arrival) can be determined within fractions of a
beamwidth by comparing measurements received from two or more
simultaneous beams. This technique for estimating the direction of
arrival (DOA) of a target is often implemented in surveillance and
tracking radar systems comprising a phased array antenna and a
digital beamforming (DBF) processor. Typically, one beam is formed
for transmission and two beams are formed upon reception for angle
measurement. The term monopulse refers to the fact that the echo
from a single transmitted pulse returning from a target is used to
measure the angle of the target.
[0013] Monopulse processing may be implemented for a linear array
of N antenna elements which provides respective signals x(0), . . .
,x(N-1) to a beamforming network. The output signals of the
beamforming network are the sum, .SIGMA., and difference, .DELTA.,
signals which are processed to generate an output signal, .theta.,
representing the estimated direction of arrival. The sum beam
pattern has a symmetrical amplitude profile with respect to its
maximum at the boresight, and the difference beam pattern has an
antisymmetrical amplitude profile with respect to a zero response
at the boresight. In the beamforming network, each of the N input
signals is split into two paths, linearly weighted, and then added
together. The DOA of a target signal is determined by evaluating
(e.g., from a look up table or from a graph) the ratio of the
difference signal over the sum signal, as indicated by the
following equation. 1 m = ( 1 )
[0014] where m is referred to as the monopulse ratio.
[0015] Monopulse processing may also be implemented for planar
arrays in which the target azimuth and elevation angles are of
interest. In this case the sum and difference signals represent sum
and difference signals for angles in elevation and azimuth. These
angles are represented by the following symbols: .DELTA..sub.E
represents the difference signal in elevation, .DELTA..sub.A
represents the difference signal in azimuth, .SIGMA. represents the
sum signal in elevation as well as in azimuth.
[0016] In an exemplary embodiment of the invention, the accuracy of
the monopulse ratio, m, is maintained in the presence of multiple
mainlobe and multiple sidelobe interference (e.g., caused by
multiple jammers). The accuracy of the monopulse ratio, m, is
maintained by adaptively suppressing the interference before
forming the monopulse sum and difference output beams. Sub-arrays
are formed using digital beamforming, DBF. Beam patterns are formed
for each sub-array and jamming interference is cancelled for each
sub-array by steering beam pattern nulls at the interference. The
sub-array beam patterns are then used to form the sum, .SIGMA., and
difference, .DELTA., beams to determine the target DOA. Thus,
unbiased angle estimation is achieved by exploiting the available
degrees of freedom across the aperture while jamming is canceled in
the sub-arrays.
[0017] FIG. 1 is a block diagram of an exemplary monopulse
sub-array sum-difference beamformer 100. Antenna array data are
digitally summed by summer 2, to form a number, N, of overlapped
sub-arrays. The amount of overlap may vary and is determined by
factors such as the beamwidth of the mainlobe and available degrees
of freedom in the aperture. Monopulse processing performance, as
measured by the monopulse ratio, m, is determined, in part, by
sub-array separation. In an exemplary embodiment of the invention,
the array is partitioned into quadrants for forming respective
quadrant beams in sub-array beamformer 4. The sub-array quadrant
beams are denoted by Q.sub.1(T.sub.x,T.sub.y),
Q.sub.2(T.sub.x,T.sub.y), Q.sub.3(T.sub.x,T.sub.y), and
Q.sub.4(T.sub.x,T.sub.y), located at (D.sub.x,D.sub.y),
(-D.sub.x,D.sub.y), (-D.sub.x,-D.sub.y), and (D.sub.x,-D.sub.y),
respectively, with respect to the center of the array. T.sub.x and
T.sub.y denote the conventional directional cosines, representing
azimuth angle (.theta..sub.AZ) and elevation angle (.theta..sub.EL)
information, respectively. Thus:
Tx=cos(.theta..sub.EL)sin(.theta..sub.AZ), (2)
[0018] and
Ty=sin(.theta..sub.EL). (3)
[0019] The quadrant beams differ from each other in phase. The
quadrant beams are represented by the following equations. 2 Q 1 (
T x , T y ) = Q ( T x , T y ) j 2 ( T x D x + T y D y ) ( 4 ) Q 2 (
T x , T y ) = Q ( T x , T y ) j 2 ( - T x D x + T y D y ) ( 5 ) Q 3
( T x , T y ) = Q ( T x , T y ) j 2 ( - T x D x - T y D y ) ( 6 ) Q
4 ( T x , T y ) = Q ( T x , T y ) j 2 ( T x D x - T y D y ) ( 7
)
[0020] where Q(T.sub.x,T.sub.y) is a common sub-array beam located
at the center of the array, .lambda. is the wavelength of the
transmitted radar pulse, and D.sub.x is the distance between the
center of the sub-array and the center of the array in the x
direction (azimuth), in meters, and D.sub.y is the distance between
the center of the sub-array and the center of the array in the y
direction (elevation), in meters. The quadrant beams are summed, by
the monopulse sum-difference beamformer 6, to form the sum,
.SIGMA.(T.sub.x,T.sub.y), azimuth difference,
.DELTA..sub.A(T.sub.x,T.sub.y), and elevation difference,
.DELTA..sub.E(T.sub.x,T.sub.y), outputs in accordance with the
following equations. 3 ( T x , T y ) = Q 1 ( T x , T y ) + Q 2 ( T
x , T y ) + Q 3 ( T x , T y ) + Q 4 ( T x , T y ) = Q ( T x , T y )
4 cos ( 2 T x , T y ) 4 cos ( 2 T x D x ) cos ( 2 T y D y ) ( 8 ) A
( T x , T y ) = Q 1 ( T x , T y ) - Q 2 ( T x , T y ) - Q 3 ( T x ,
T y ) + Q 4 ( T x , T y ) = Q ( T x , T y ) 4 j sin ( 2 T x , D x )
cos ( 2 T y D y ) ( 9 ) E ( T x , T y ) = Q 1 ( T x , T y ) + Q 2 (
T x , T y ) - Q 3 ( T x , T y ) - Q 4 ( T x , T y ) = Q ( T x , T y
) 4 j cos ( 2 T x D x ) sin ( 2 T y D y ) . ( 10 )
[0021] The azimuth monopulse ratio, m.sub.A, and the elevation
monopulse ratio, m.sub.E, are calculated in accordance with the
following equations by monopulse ratio former 34. 4 m A = A ( T x ,
T y ) ( T x , T y ) = Q ( T x , T y ) j4 sin ( 2 T x D x ) cos ( 2
T y D y ) Q ( T x , T y ) 4 cos ( 2 T x D x ) cos ( 2 T y D y ) = j
tan ( 2 T x D x ) ( 11 ) m E = E ( T x , T y ) ( T x , T y ) = Q (
T x , T y ) j4 cos ( 2 T x D x ) sin ( 2 T y D y ) Q ( T x , T y )
4 cos ( 2 T x D x ) cos ( 2 T y D y ) = j tan ( 2 T y D y ) ( 12
)
[0022] In an exemplary embodiment of the invention, adaptive
processing techniques are implemented to reduce interference due to
jamming. Various adaptive processing techniques may be implemented
depending upon the number of available antenna element data.
Examples of adaptive processing techniques include a main auxiliary
adaptation technique, an adaptive-adaptive processing technique,
and a fully adaptive array technique.
[0023] In an exemplary embodiment of the invention, gain at the
center of the main beam is maintained during the application of
each of these techniques. In the main auxiliary adaptation
technique, main beam and auxiliary beams (beams having
approximately omnidirectional beam patterns and relatively low
gain) are formed from available array elements, which are capable
of being shared among multiple beams. The auxiliary beams are used
to cancel the sidelobe jamming in the sum beam.
[0024] In the adaptive-adaptive processing technique, the auxiliary
beams are steered in the direction of the jammers. As is the case
in the main auxiliary adaptation technique, the auxiliary beams are
used to cancel jamming in the sum beam. In the fully adaptive array
technique, all elements of the array are used to cancel jamming
while the sum beam is formed from all elements of the array.
[0025] For example, in the context of fully adaptive array
processing, in which all antenna element data are available, the
adaptive processing can be formulated using the sub-array degrees
of freedom (i.e., the number of elements available in the
sub-array) to cancel jamming subject to the constraint of sub-array
bore-sight gain. To reduce jammer interference, the jammer power is
minimized. To reduce jammer interference and maintain target
detection, jammer power is minimized subject to a constraint of
maintaining sub-array boresight gain. Jammer power is given by the
equation:
J.sub.1=W.sub.1.sup.HR.sub.11W.sub.1, (13)
[0026] where J.sub.1 is the received power of the jammer, W.sub.1
is the adaptive weight for sub-array number 1, and R.sub.11 is the
covariance matrix measurement of the first sub-array, and the
superscript H indicates the complex conjugate transpose. The
constraining equation is:
S.sub.1.sup.HW.sub.1=g.sub.1, (14)
[0027] where S.sub.1 is the sub-array steering vector, and g.sub.1
is the bore-sight sub-array gain. Thus, combining equations (13)
and (14) results in the following equation represent jammer power
subject to the constraint of maintaining sub-array boresight
gain.
J.sub.1=W.sub.1.sup.HR.sub.11W.sub.1-.alpha.(S.sub.1.sup.HW.sub.1-g.sub.1)-
, (15)
[0028] where .alpha. is the LaGrange multiplier for constrained
optimization.
[0029] Minimizing equation (15) with respect to W.sub.1 (e.g.,
equating the first derivative of J.sub.1 with respect to W.sub.1 to
zero) and solving for W.sub.1 results in the following equation for
the adaptive weights for the sub-array, when the constraint of
equation (15) is observed. 5 W 1 = R 11 - 1 S 1 S 1 H R 11 - 1 S 1
g 1 ( 16 )
[0030] The adaptive weights for the other sub-arrays are derived in
a similar manner. Thus, 6 W 2 = R 11 - 1 S 2 S 2 H R 33 - 1 S 3 g 2
( 17 ) W 3 = R 33 - 1 S 3 S 3 H R 33 - 1 S 3 g 3 and ( 18 ) W 4 = R
44 - 1 S 4 S 4 H R 44 - 1 S 4 g 4 . ( 19 )
[0031] FIG. 2 is a block diagram of an exemplary system 200
including adaptive digital sub-array beamformer 8 and monopulse
processor 34, with jamming cancellation and monopulse ratio
preservation, in accordance with the present invention. Antenna
element data are formed into sub-arrays by summer 2, and the
sub-array data are provided to adaptive sub-array beamformer 8.
Adaptive sub-array beamformer 8 forms beam patterns for each
sub-array, with nulls adaptively located (steered) to suppress
jamming interference. The nulls may be steered in the direction of
jamming interference as a result of the adaptation. Adapted of
estimated values are represented by placing a circumflex
("{circumflex over ( )}") over the estimated variable or quantity.
The adaptive beam pattern formed from elements of each sub-array
result in identical nulls responsive to mainlobe or sidelobe
jammers. The adaptive sub-array beams can also be related to a
common adaptive sub-array beam located at the center of the array
({circumflex over (Q)}(T.sub.x,T.sub.y)). In the exemplary
situation where quadrant beams are formed, the estimated quadrant
beams are represented by the following equations. 7 Q ^ 1 ( T x , T
y ) = Q ^ ( T x , T y ) j2 ( T x D x + T y D y ) ( 20 ) Q ^ 2 ( T x
, T y ) = Q ^ ( T x , T y ) j2 ( - T x D x + T y D y ) ( 21 ) Q ^ 3
( T x , T y ) = Q ^ ( T x , T y ) j2 ( - T x D x - T y D y ) ( 22 )
Q ^ 4 ( T x , T y ) = Q ^ ( T x , T y ) j2 ( T x D x - T y D y ) (
23 )
[0032] where equations (20), (21), (22), and (23) are analogous to
equations (4), (5), (6), and (7), respectively, wherein the beam
patterns represented by equations (20), (21), (22), and (23) are
adaptively estimated.
[0033] The estimated sub-array beams are provided to the monopulse
sum and difference beamformer 10. Monopulse sum and difference
beamformer 10 provides estimated sum, and difference beams in
elevation and azimuth, {circumflex over
(.SIGMA.)}(T.sub.x,T.sub.y), {circumflex over
(.DELTA.)}.sub.A(T.sub.x,T.sub.y), and {circumflex over
(.DELTA.)}.sub.A(T.sub.x,T.sub.y), respectively, to monopulse ratio
former 34. Monopulse ratio former 34 calculates the monopulse ratio
in azimuth and elevation in accordance with the following
equations. Thus leading to the following undistorted estimated
monopulse ratios given by: 8 m ^ A = ^ A ( T x , T y ) ^ ( T x , T
y ) = Q ^ ( T x , T y ) j4 sin ( 2 T x D x ) cos ( 2 T y D y ) Q ^
( T x , T y ) 4 cos ( 2 T x D x ) cos ( 2 T y D y ) = j tan ( 2 T x
D x ) , ( 24 ) m ^ E = ^ E ( T x , T y ) ( T x , T y ) = Q ^ ( T x
, T y ) j4 cos ( 2 T x D x ) sin ( 2 T y D y ) Q ^ ( T x , T y ) 4
cos ( 2 T x D x ) cos ( 2 T y D y ) = j tan ( 2 T y D y ) , ( 25
)
[0034] where {circumflex over (m)}.sub.A is the estimated monopulse
ratio in azimuth and {circumflex over (m)}.sub.E is the estimated
monopulse ratio in elevation.
[0035] Thus, the jamming is cancelled by the adaptive sub-array
beamformer 8, using the spatial degrees of freedom of each
sub-array (i.e., the number of available elements in each
sub-array) to steer at least one null of each sub-array beam toward
the jamming interference. Further, the spatial degrees of freedom
across the sub-arrays (i.e., the number of available elements in
the total array) are used to form conventional monopulse beams for
angle estimation in monopulse sum and difference beamformer 10.
[0036] FIG. 3 is a block diagram of an exemplary adaptive digital
sub-array beamformer and monopulse processor, with jamming
cancellation and monopulse ratio preservation, for a
one-dimensional array, in accordance with the present invention.
Processing for the one-dimensional array is similar to that of the
two-dimensional array, except that the beam patterns are in one
less dimension in the one-dimensional array implementation than in
the two-dimensional array implementation. Also, the estimated
adaptive processing is applied to sub-arrays which differ by a
linear phase factor. Sub-array data are provided to one-dimensional
adaptive sub-array beamformer 12. Estimated sub-array beams (i.e.,
A.sub.1(T.sub.x), A.sub.2(T.sub.x), A.sub.3(T.sub.x), and A.sub.4
(T.sub.x)) are formed by one-dimensional adaptive sub-array
beamformer 12, in which the nulls are adaptively located to
suppress jammer interference. The nulls may be steered in the
direction of jamming interference as a result of adaptation. The
estimated sub-array beams are provided to monopulse sum and
difference beamformer 14. Monopulse sum and difference beamformer
14 forms estimated sum and difference beams (i.e., {circumflex over
(.SIGMA.)}(T.sub.x) and {circumflex over (.DELTA.)}(T.sub.x)),
which are use to calculate the monopulse ratio, {circumflex over
(m)}, in monopulse ratio former 36, in accordance with the
following equation. Thus the unbiased monopulse angle estimate is
given by the following equation. 9 m ^ = ^ ( T x ) ^ ( T x ) = S ^
A ( T x ) j 4 sin ( 2 T x D x ) cos ( 2 T x D x ) S ^ A ( T x ) 4
cos ( 2 T x D x ) cos ( 2 T x D x ) = j tan ( 2 T x D x ) , ( 26
)
[0037] where D.sub.x is the distance between the center of the sub
array and the center of the array in the x direction, in
meters.
[0038] A radar processing system and technique utilizing the
versatility of digital beamforming to form sub-arrays for canceling
jamming interference, and adaptively suppressing jamming
interference prior to using conventional deterministic methods
(i.e., equations (24) and (25)) to form the sum, .SIGMA., and
difference, .DELTA., beams for monopulse processing provide the
ability to detect a target of interest, provide an undistorted
monopulse ratio, m, and maintain target angle estimation, in the
presence of multiple mainlobe and multiple sidelobe jammers.
Further, this system and technique are not constrained by requiring
a priori knowledge of the jamming interference.
[0039] FIG. 4 is a flow diagram of an exemplary process for
detecting a target of interest and maintaining a monopulse ratio in
the presence of multiple mainlobe jammers and multiple sidelobe
jammers in accordance with the present invention. Sub-arrays are
formed using digital beamforming, from the full antenna array in
step 16. Antenna array element data are digitally summed to form
the sub-arrays. Step 16 may be performed by summer 2 of FIG. 1. The
amount that the sub-arrays overlap (share the same antenna element
data) is discretionary. Factors to consider when determining which
antenna elements are assigned to the various sub-arrays include the
desired beamwidth of each sub-array, the degrees of freedom
available for adaptive processing, and the impact on monopulse
processing performance parameters.
[0040] In step 18, sub-array beams are adaptively formed for each
sub-array. Each sub-array beam pattern comprises a mainlobe and at
least one null. In an exemplary embodiment of the invention, four
quadrant sub-arrays are formed. Nulls are adaptively formed to
suppress jammer interference by multiplying sub-array element data
by the adaptively formed weights formed in accordance with
equations (16), (17), (18), and (19). The adaptively formed
sub-array beam patterns are in accordance with equations (20),
(21), (22), and (23). At least one null of each sub-array beam
pattern is adaptively steered toward the jamming interference,
while the boresight gain of each sub-array beam pattern is
maintained. Monopulse sum, and difference beams in elevation and
azimuth, {circumflex over (.SIGMA.)}(T.sub.x,T.sub.y), {circumflex
over (.DELTA.)}.sub.E(T.sub.x,T.sub.y), and {circumflex over
(.DELTA.)}.sub.A(T.sub.x,T.sub.y), respectively, are determined
using the full antenna array element data in step 24. Monopulse
ratios are determined in step 26, in accordance with equations (24)
and (25), to provide undistorted monopulse ratios {circumflex over
(m)}.sub.A and {circumflex over (m)}.sub.E. The monopulse ratios
are used to determine the estimated angle of arrival of the target
of interest. This may be accomplished through the use of a look up
table or from extracting data from a plot of monopulse ratio versus
arrival angle. The monopulse ratio is maintained in step 28 by
updating the sub-array beam patterns to steer the null(s) toward
the jamming interference. The sum and difference beams are
recalculated using the updated beam patterns, and updated monopulse
ratios are calculated to maintain the accuracy of the estimated
arrival angle.
[0041] The present invention may be embodied in the form of
computer-implemented processes and apparatus for practicing those
processes. FIG. 5 is a block diagram of a radar system comprising
an antenna array 30 and computer processor 32 in accordance with an
exemplary embodiment of the invention. The exemplary array 30 has
four sub-arrays 34A, 34B, 34C, and 34D, each sub-array including a
plurality of antenna elements 36. Data received by antenna array 30
is transmitted to computer processor 32. Computer processor 32
performs processes for detecting a target of interest and
maintaining a monopulse ratio in the presence of multiple mainlobe
jammers and multiple sidelobe jammers in accordance with the
present invention, as herein described with reference to FIG. 4.
Received signal processing may also be performed by special purpose
hardware.
[0042] The present invention may also be embodied in the form of
computer program code embodied in tangible media, such as floppy
diskettes, read only memories (ROMs), CD-ROMs, hard drives, high
density disk, or any other computer-readable storage medium,
wherein, when the computer program code is loaded into and executed
by computer processor 32, the computer processor 32 becomes an
apparatus for practicing the invention. The present invention may
also be embodied in the form of computer program code, for example,
whether stored in a storage medium, loaded into and/or executed by
computer processor 32, or transmitted over some transmission
medium, such as over electrical wiring or cabling, through fiber
optics, or via electromagnetic radiation, wherein, when the
computer program code is loaded into and executed by computer
processor 32, the computer processor 32 becomes an apparatus for
practicing the invention. When implemented on a general-purpose
processor, the computer program code segments configure the
processor to create specific logic circuits.
[0043] Although illustrated and described herein with reference to
certain specific embodiments, the present invention is nevertheless
not intended to be limited to the details shown. Rather, various
modifications may be made in the details within the scope and range
of equivalents of the claims and without departing from the spirit
of the invention.
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